Many tumor-reactive monoclonal antibodies have been developed but most have little anti-tumor activity. Often this can be overcome by using multimeric forms of the antibody which are then able to induce death signals in the cell by hypercrosslinking their target antigen. In addition, increasing their in vivo half-life to maximize tumor penetration is often helpful. Although chemically-generated tetravalent antibodies are highly effective in vitro, they have poor pharmacokinetics due to their large size. They are also difficult and expensive to produce and consist of heterogeneous mixtures of products. Recombinant multivalent constructs comprised of several Fv fragments have no effector functions and short half lives in vivo. With regard to the Fc region of an antibody, binding to FcRns in endothelial cells is critical for prolonging their in vivo half life. Hence, specific mutations in the Fc region of an IgG can increase or decrease FcRn binding and thereby increase or decrease its serum half-life. The goal of this study is to construct a panel of antibodies that 1) are tetravalent and contain a human Fc region in order to enhance their ability to induce apoptosis and to decrease immunogenicity, 2) have sizes comparable to IgGs for good tumor penetration, and 3) incorporate FcRn-mutations, to alter their half life in vivo. Constructs with both longer and shorter half-lives will be evaluated in vitro and in vivo in mice with human tumors. The former will be used as "naked" antibodies where a long half-life should be advantageous. The latter will be used as immunotoxins, where a short half-life may reduce toxic side effects, thereby increasing the maximum tolerated dose.